(1) Field of the Invention
This invention relates to phase shifting masks used to form photolithographic images on wafers and more particularly to mask corrections to compensate for image distortion.
(2) Description of the Related Art
Photolithography is critical to the fabrication of integrated circuit wafers and utilizes masks to transfer images, such as line/space patterns, to the wafer. As circuit densities increase critical dimensions decrease and line/space patterns become more and more dense. FIG. 1 shows an example of a part of a dense line/space pattern on a wafer with a number of lines 20 having a first linewidth 22 on a first pitch 24. FIG. 2 shows an example of a part of an isolated line/space pattern with lines 21 having a second linewidth 23, greater than the first linewidth 22, on a second pitch 25, greater than the first pitch 24. Frequently both dense line/space patterns and isolated line/space patterns are part of the same circuit pattern formed on a wafer.
FIG. 3 shows a part of a binary mask, often used in the formation of the patterns on a wafer, having opaque lines 32 formed on a transparent mask wafer 30. FIG. 4 shows an alternative phase shifting mask, sometimes referred to as a Levenson phase shifting mask, used as an alternative to a conventional binary mask for forming line/space patterns. The alternative phase shifting mask has lines 34 formed of phase shifting material on a transparent mask substrate 30.
In forming photolithographic patterns exposure latitude and depth of focus are important parameters. As the line/space patterns become more dense optical distortion becomes a problem in the pattern formation. Optical proximity effect is a form of optical distortion associated with the formation of photolithographic images. Diffraction effects occurring on both sides of a sharp pattern edge become important as the critical dimensions of pattern features decreases. The use of alternative phase shifting masks in place of binary masks improves the exposure latitude and depth of focus but the problem of proximity effect increases.
U.S. Pat. No. 5,858,591 to Lin et al. describes a method of correcting for optical distortion during wafer processing by modification of subfiles used to form the masks. The subfiles are modified to correct for optical proximity effect and to add or subtract a suitable bias to the subfiles.
U.S. Pat. No. 5,682,323 to Pasch et al. describes a system and method of performing optical proximity correction on an integrated circuit by performing optical proximity correction on a library of cells.